1. Field of the Invention
The present invention relates to an image forming apparatus that is capable of detecting the overlapped feeding of regular sheets and insertion sheets.
2. Description of the Related Art
Hitherto, in an image forming apparatus such as a copying machine, a mode (hereinafter referred to as “insertion mode”) for inserting a front cover, a back cover, or an intermediate insertion sheet (hereinafter referred to as “insertion sheet”) is provided. In the insertion mode, insertion of the front cover, the back cover, or an intermediate insertion sheet may be performed by controlling the apparatus to feed the first page, the last page, or the predetermined page in its correct position from a predetermined feed unit, and to feed other regular pages from a paper feed unit different from the predetermined feed unit.
Provision of the predetermined feed unit for feeding the insertion sheet at the sheet processing unit (finisher) to be mounted on the image forming apparatus enables insertion of the insertion sheet without causing a damage thereto by a heat and pressure because the insertion sheets are fed without passing the front cover and the like having an image formed thereon in advance through the image forming apparatus, or without passing them through a fixing unit in an electrophotographic image forming apparatus. The sheet processing unit for feeding the insertion sheet is set forth in Japanese Patent Laid-Open No. 60-180894, Japanese Patent Laid-Open No. 60-191932, and Japanese Patent Laid-Open No. 60-204564.
When feeding the insertion sheets that are pre-printed with images, a front cover, an intermediate insertion sheet, and a back cover are stacked in order and set in the sheet processing unit. Therefore, when feeding the insertion sheets that are pre-printed with images, the insertion sheets fed from the feed unit must be reliably fed into the sheet processing unit one by one.
However, since various types of paper are used for the insertion sheets, there may be a case in which feeding of two, instead of one, sheets simultaneously, so called overlapped feed, may occur, which causes a problem that the order of the sheets to be fed from then on may be out of sequence.